The present invention relates to a process for forming a well, a preflush composition for use in the well forming process, and a well configuration.
One or more air or lost circulation zones can be penetrated during a well drilling operation. Lost circulation zones are highly permeable formations. An exemplary lost circulation zone consists mostly of unconsolidated pebbles without any substantial clay sedimentation. When a casing is set in a borehole that penetrates an lost circulation zone, a substantial amount of cement slurry used to set the casing in the borehole is generally lost to the lost circulation zone. In some instances involving lost circulation zones, no matter how much cement is poured into the borehole through the casing, the cement fails to reach the surface of the borehole on the annulus side of the casing. In other instances, although the cement slurry initially reaches the surface of the borehole, the cement slurry excessively recedes into the borehole. In the latter situations, cement has been observed to slip over about 150 feet back into the borehole.
A remedial cementing job is usually necessary for wells constructed through these lost circulation zones. The loss of cement slurry to the lost circulation zone and the remedial cementing jobs adds to the cost of the drilling operation. Furthermore, even after the remedial cementing work has been performed, casing expansion and contraction during well operations, e.g., steam injection and shut-down, are serious problems.
Cement slurries containing various additives have been employed in an attempt to overcome the above problems. However, the use of additive-containing cement slurries has at least two disadvantages. First, additive-containing cement slurries are more expensive than the conventional cement slurries used in well drilling operations. In addition, some additives change the cement slurry's rheology characteristics and make the resulting slurry more difficult to pump. Other additives can detrimentally affect the slurry's setting rate or the compressive strength of the resulting cement.